1. Field of the Invention
The present invention relates to a method for subbing polyester films and, more particulary, the invention relates to a method for subbing polyethylene terephthalate films (hereinafter, referred to as PET films) as supports for photographic materials.
2. Description of the Prior Art
Polyester films, in particular, PET films possess various features such as high mechanical strength, excellent dimensional stability, etc., as compared with other plastic films and hence the fields of applications of these films have greatly increased. Particularly, the demand for these films as supports for photographic materials has grown rapidly.
However, because polyester films are generally highly crystalline, are chemically inactive, have high chemical resistance, and are very hydrophobic, it is difficult to strongly adhere a hydrophilic organic colloid layer, such as a photographic silver halide emulsion layer, to the polyester film. In general there are various methods of strongly adhering a hydrophilic organic colloid layer to a polyester film. For example, the surface of a polyester film may be subjected to a surface activation treatment such as ultraviolet irradiation and then coated with a solution of hydrophilic resin using a solvent for the polyester or using a mixed organic solvent containing a swelling agent or the polyester surface may be coated with a liquid subbing composition comprising a gelatin dispersion. In another method a vinylic copolymer layer comprising vinylidene chloride, vinyl chloride, styrene, an acrylic acid ester, acrylic acid, etc., is formed on the surface of a polyester film and then a subbing composition comprising an aqueous gelatin solution or an aqueous gelatin dispersion is coated thereon.
On the other hand, the adhesive strength of a hydrophilic organic colloid layer after forming a subbing layer on a polyester film and then forming thereon the hydrophilic organic colloid layer is generally evaluated on two basis. One is the adhesive strength (hereinafter, referred to as the adhesive strength in wet state) when the photographic film thus obtained is placed in an aqueous solution such as, for example, a photographic developer, etc. The other is the adhesive strength (hereinafter, referred to as the adhesive property in dry state) in the dry state of the photographic film thus obtained. However, generally if the adhesive property of a hydrophilic organic colloid layer to the polyester film is insufficient in the dry state, it is sufficient in wet state and, in contrast with this, if the adhesive property is insufficient in wet state, it is sufficient in dry state.
Therefore, it is quite difficult to satisfy the both cases and as a manner for overcoming the difficulty, a method wherein a hardening agent is incorporated in the subbing gelatin layer has been adopted.
Hitherto, as the hardening agent for the subbing layer, formalin, chromium complex salts, ethyleneimine compounds, etc., have been mainly used. It is desired that the hardening agent used for this purpose is highly soluble in an aqueous solution or an organic solvent, does not react with gelatin in solution but reacts immediately after coating without having any chemical influence on the photosensitive property of the photographic silver halide emulsions, and further has no toxicity to human body.
If the reactivity of a hardening agent is too high, the reaction thereof in solution proceeds fast and causes deterioration of the coating composition with the passage of time, while if the reactivity is too slow, a heat treatment at a comparatively high temperature is required after coating, which sometimes results in reducing the flatness of the support.